Thyristor voltage switching circuits

Abstract

Improved thyristor voltage switching circuits for connecting an a.c. voltage across a load are characterized by means for protecting the thyristors when in the off state against line voltage transients of an amplitude in excess of breakdown voltages thereof. In one embodiment of the invention, the thyristors are triacs connected in series to increase their overall voltage withstand capability. In another embodiment, circuitry is connected to sense the voltage between anode and cathode terminals of triacs, and to apply a signal to gate terminals thereof to render the triacs conductive whenever the anode to cathode voltage is in excess of the breakdown voltage. In this manner, surge currents are conducted across the entire junction areas of the triacs as compared with point areas, and destruction of the triacs is prevented.

What is claimed is: 
     
       1. In a voltage switching circuit, first and second semiconductor devices each having a pair of controlled electrodes, a control electrode for controlling conduction between said controlled electrodes, and a breakdown voltage rating between said controlled electrodes, one of said controlled electrodes of one said devices being connected with one of said controlled electrodes of the other said devices connecting said devices in series, the other controlled electrodes of said devices for being connected across a load and a source of voltage having a nominal value less than the combined value of said breakdown voltages, and means for sensing the voltage across said other controlled electrodes of said devices and for applying a signal to said control electrodes to establish conduction between said controlled electrodes of said devices whenever said voltage exceeds a predetermined value greater than said nominal value and less than the combined value of said breakdown voltages. 
     
     
       2. In a voltage switching circuit as set forth in claim 1, said control electrode of each of said first and second devices being coupled through said device to said other controlled electrode thereof and having a voltage thereat substantially equal in value to the voltage at said other controlled electrode, said sensing and applying means being a third semiconductor device having a pair of terminals and responsive to a voltage thereacross to establish a conductive path therebetween whenever said voltage is in excess of said predetermined value, said third semiconductor device being connected by said terminals between said control electrodes of said first and second devices. 
     
     
       3. In a voltage switching circuit as set forth in claim 2, wherein said first and second devices are first and second thyristors each having anode, cathode and gate electrodes, said controlled electrodes being said anode and cathode electrodes and said control electrodes being said gate electrodes, said anode electrodes of said thyristors being connected together and said cathode electrodes thereof for being connected across said load and said source of voltage, said terminals of said third device being connected between said gate electrodes. 
     
     
       4. In a voltage switching circuit as set forth in claim 3, said first and second thyristors being first and second triacs having said anode, cathode and gate electrodes, said third semiconductor device being a metal oxide varistor having a conduction voltage equal in value to said predetermined value. 
     
     
       5. In a voltage switching circuit as set forth in claim 4, means connected between said gate electrodes of said triacs for controllably establishing a circuit path therebetween, said triacs connecting said voltage across said load when said circuit path is established. 
     
     
       6. In a voltage switching network for selectively connecting an a.c. voltage having a nominal value across a load, said voltage occasionally having imposed thereon transient voltages of a value greater than said nominal value, first and second triacs each having anode, cathode and gate electrodes and a nonconductive breakdown voltage between said anode and cathode electrodes of a value greater than one-half said nominal value, said anode electrodes being connected together and said cathode electrodes for being connected across said load and said voltage, and a metal oxide varistor having a pair of terminals responsive to a voltage thereacross to establish a conductive path therebetween whenever said voltage is in excess of a predetermined value greater than said nominal value and less than the combined breakdown voltage values, connected by said terminals between said triac gate electrodes for having said a.c. voltage applied across said terminals, whereby upon the occurrence of a voltage across said cathode electrodes of greater value than said predetermined value said metal oxide varistor conducts to interconnect said triac gates to render said triacs conductive and to thereby limit said voltage across said triacs to less than said breakdown value. 
     
     
       7. In a voltage switching circuit as set forth in claim 6, means connected between said triacs gate electrodes for controllably establishing a circuit path therebetween whereby said triacs connect said a.c. voltage across said load when said circuit path is established.